The invention relates to a device for reading out the charge condition of a phototransistor by means of sampling pulses as an example, read-out of a transistor to be selected from a matrix of phototransistors is sampled by the sampling pulses, and a voltage which is a measure of the charge condition is taken from the emitter of the phototrasnsistor.
Such a device is known from "I.E.E.E. Transactions on Electron Devices", Vol. ED-15, No. 4, April 1968, pages 226 - 237. In this device a sampling voltage pulse is applied to the collector of the phototransistor to be read out and a signal is taken from the emitter across a load resistor. At the instant that the sampling pulse appears, this signal exhibits a switching transient of substantially the same height as the sampling pulse. Subsequently, the voltage across the resistor decreases with a time constant which is mainly determined by the value of the load resistor and the capacitance of the cut-off base-emitter junction. At the instant that the base-emitter junction is turned on, the transistor discharges with a time constant which is mainly determined by the capacitance of the (reverse biased) base collector junction and the sum of the values of the load resistor and the differential resistance of the base-emitter junction. The instant at which the base-emitter junction is turned on, which is apparent from the changed time constant, is determined by the charge which is generated in the transistor by the incident light.
Drawbacks of the prior-art device are that the differential resistance of the base-emitter junction, and thus the discharge time constant, increases substantially as the transistor is further discharged. This last-mentioned phenomenon results in a storage effect because the transistor, especially in case of very short sampling pulses, is not discharged sufficiently. As a result of this, the transistor is further discharged upon a subsequent sampling pulse, even if no charge is produced in the intermediate period owing to a dark period.